Synthesis, characterization and in vivo studies of Cu(II)-64-labeled cross-bridged tetraazamacrocycle-amide complexes as models of peptide conjugate imaging agents

Copper-64, a positron emitter suitable for positron emission tomography (PET), demonstrates improved in vivo clearance when chelated by the cross-bridged tetraazamacrocycle CB-TE2A compared to TETA. Good in vivo clearance was also observed for ⁶⁴Cu-CB-TE2A conjugated to a peptide, which converts one coordinating carboxylate pendant arm to an amide. To better understand the in vivo stability of peptide-conjugated CB-TE2A, cross-bridged monoamides were synthesized. Crystal structures of ^natCu(II)-CB-TEAMA and ^natCu(II)-CB-PhTEAMA revealed hexadentate, distorted octahedral coordination geometry. In vivo biodistribution showed clearance of all ⁶⁴Cu-radiolabeled cross-bridged monoamides from liver and bone marrow such that uptake at 24 h was <10% of uptake at 30 min. In contrast, >60% of 30 min uptake from ⁶⁴Cu-TETA was retained in these tissues at 24 h. Clearance of ⁶⁴Cu-cross-bridged monoamides from nontarget organs suggests good in vivo stability, thus supporting the use of CB-TE2A as a bifunctional chelator without modifications to the macrocycle backbone.